This invention relates to solar cells, and, more particularly, to the fabrication of lightweight solar cells and their interconnection.
Semiconductor solar cells are utilized to convert light energy to useable electrical voltages and currents. Briefly, a typical semiconductor solar cell includes an interface between n-type and p-type transparent semiconductor materials. Light shining on the semiconductor materials adjacent the interface creates hole-electron pairs in addition to those otherwise present, and the minority charge carriers migrate across the interface in opposite directions. There is no compensating flow of majority carriers, so that a net electrical charge results. A useful electrical current is obtained in an external electrical circuit by forming ohmic contacts to the materials on either side of the interface.
In general terms, a photovoltaic solar cell is fabricated by depositing the appropriate semiconductor layers onto a substrate, and then adding additional components to complete the cell. As an example, a conventional p-on-n gallium arsenide solar cell is fabricated by epitaxially depositing a layer of n-type gallium arsenide onto a single crystal gallium arsenide substrate, and depositing a layer of p-type gallium arsenide overlying the layer of n-type gallium arsenide. The interface between the p-type gallium arsenide and the n-type gallium arsenide forms the basic solar cell active structure. External ohmic electrical contacts to the n-type and p-type layers are applied, and a voltage is measured across the contacts when light energy is directed against the interface. Optionally, a P+layer of gallium aluminum arsenide may be deposited over the layer of p-type gallium arsenide to limit recombination of charge carriers. To protect the solar cell from physical contact and radiation damage such as encountered in a space environment, it is conventional to apply a transparent cover of glass over the solar cell components.
The solar cells are connected together into large arrays to deliver power of the desired voltage and current. A typical earth satellite such as a Hughes Aircraft Co. HS-376 communications satellite may have as many as 15,000 solar cells, each about 1 inch by 21/2 inches in size. Since the cost of raising weight to orbit is high, the weight of the cells, and their associated hardware such as the electrical interconnects between solar cells in the array, should be reduced as much as possible.
The weight of each solar cell is determined in part by the materials chosen. Silicon solar cells are now commonly used. Gallium arsenide is considerably heavier than silicon, but gallium arsenide cells are of interest for spacecraft power applications because of their greater power output per unit surface area of cell. A typical silicon solar cell is about 0.008 inches thick, and silicon solar cells as thin as about 0.002 inches have been prepared. A substantial weight saving is realized if the thickness of the cell is reduced. In principle, the cells can be made quite thin, but in practice very thin cells have been impractical because, once the cell itself is fabricated, it is very difficult to attach external electrical contacts to solar cells. Gallium arsenide and related compounds are rather brittle, and attempts to attach electrical cell contacts to a thin cell often result in cracking of the cell. If a cell less than about 0.001 inch thick can be fabricated, then in most cases the solar cell will be broken during the attempt to attach the cell contacts. It has not been heretofore possible to achieve the objective of fabricating very thin solar cell of brittle materials that can be readily joined into arrays, in the scale of production required to build satellites.
Moreover, the present approach to joining solar cells in large arrays is cumbersome. The present approach to interconnecting solar cells in series, as must be done to achieve the necessary voltages for powering spacecraft systems, uses a "Z-type" connector that extends from the underside of one cell to the top surface of the adjacent cell. The web or main body of the Z-type connector lies between the adjacent solar cells, requiring that the cells be spaced apart to accommodate the connector. The Z-type connector is difficult to install, and its use makes replacement of a failed solar cell quite difficult.
There therefore exists a need for an improved solar cell assembly amenable to attachment of external electrical contacts thereto, including the contacts used to connect the solar cell to an adjacent solar cell. Alternatively stated, since the attachment of the external electrical connectors can be viewed as part of the process of fabrication of the solar cell and of solar cell arrays, there is a need for an improved fabrication process whereby a thin solar cell of brittle materials can be fabricated in a manner allowing the external electrical contacts to be made to the cell. The present invention fulfills this need, and further provides related advantages.